In our study published on October 21, 2024 in the journal Physical Review Letters, we predict the existence of a new type of black holes which would be surrounded by rings similar in shape to those of Saturn, but made up of elementary particles.
Einstein’s theory of general relativity predicted the existence of black holes: regions in space-time where gravity is so intense that nothing, not even light, can escape. Their hypothetical existence was formulated in 1916 when solutions to the mathematical equations of General Relativity describing black holes were obtained.
However, their actual existence was subsequently debated by scientists for almost a century. In 1965, the theoretical work of British mathematician Roger Penrose demonstrated that black holes are inevitably formed by the gravitational collapse of stars and it was not until the 1990s that astronomical observations, carried out by two American astrophysicists Reinhard Genzel and Andrea Ghez, reveal what appears to be a gigantic black hole at the center of our galaxy. This discovery will earn them, as well as Roger Penrose, the Nobel Prize in Physics in 2020.
Today, thanks to observations from international collaboration Event Horizon Telescopewe are able to discern the shadow of the black hole located at the center of the Milky Way.
The existence of black holes in the Universe has therefore been firmly established, but the discoveries do not stop there. Today we predict the existence of black holes of a new type, suggested by the Standard Model of fundamental forces.
The origin of black holes
It is now commonly accepted that black holes are omnipresent in our Universe. We generally distinguish two scenarios explaining their formation.
First there are black holes stellarformed by the gravitational collapse of ordinary stars at the end of their lives, that is, when they finally collapse under their own weight after having used up all their fuel. Their mass typically varies from 2-3 solar masses to tens — or even hundreds — of solar masses. After their formation, these stellar black holes can grow by absorbing surrounding matter. They can also merge with each other, with a significant emission of gravitational waves, the first detection of which was recognized by the Nobel Prize in Physics in 2017.
It is also possible that certain black holes, called primordialwere formed by the collapse of primary matter during the first second after the Big Bang. The mass of these primordial black holes can be gigantic, up to billions of solar masses for the supermassive black holes located at the centers of most galaxies. But it can also be small, of the order of planets or asteroids, concentrated in a radius of less than a centimeter! It is therefore possible that the Universe is dotted with these tiny primordial black holes, the future detection of which constitutes a major challenge for observational astrophysics. Even lighter primordial black holes would have had to evaporate very quickly according to the Hawking process and would not have survived to the present day.
Our results suggest that some of the small primordial black holes that still exist today may have a new property: being “hairy”.
Black holes “don’t have hair”
Stellar black holes keep no memory of the star that collapsed to lead to their formation, except that of its mass, its electric (or magnetic) charge, and its rotation speed. All other characteristics of their initial state (e.g. the chemical composition of the star) are completely lost during the collapse, and all black holes of the same mass, same charge, and same rotation speed are absolutely identical.
The American physicist John Wheeler illustrated this property with a phrase that has become famous: “the three blacks have no hair”, where by “hair” we mean any parameter other than mass, charge, and speed. of rotation.
This property of stellar black holes is confirmed by uniqueness theorems, while, for primordial black holes, it has been postulated as a conjecture, partially confirmed by a series of “baldness theorems”.
And yet… the beginnings of hairy black holes
Among the four fundamental forces of nature there are two, gravitation and electromagnetism, which act on the macroscopic scale and explain the structure of stellar black holes which are “bald”. The two other forces, called weak and strong, only act on the microscopic scale, inside atoms. Can these last two fundamental forces influence the structure of black holes?
The physical theories that describe these forces are quite complicated to study, and it is for this reason that physicists first focused on simplified theoretical models. It is thanks to these simplified models that the so-called black holes were discovered. hairythat is to say surrounded by a shell of material intrinsically linked to them and therefore characterized by additional parameters (other than mass, charge, rotation speed) which make it possible to distinguish them from each other.
Since their first discovery in 1989, numerous examples of hairy black holes have been found by theoretical physicists, but always within the framework of simplified or, conversely, extremely speculative theories. Such three blacks exist on paper as solutions to mathematical equations, but there is nothing to confirm that they really exist in our Universe.
Black holes with “electroweak hair”
In our study, we considered the unification of three exact, unsimplified, and experimentally confirmed theories, which bring together three of the four fundamental forces: gravitation, electromagnetism, and the weak nuclear force (the last two together form the force electroweak).
The solutions we obtained by solving the equations of these combined theories describe magnetically charged black holes surrounded by a “hair” shaped ring.
These rings are composed of elementary particles (more precisely, W, Z and Higgs bosons), in the form of a Bose-Einstein condensate — a particular state of matter appearing in certain situations. In the laboratory, it was observed for cold atoms trapped using lasers (which earned its discoverers a Nobel Prize in 2001). In our case, it is the intense magnetic field of the charged black hole that produces the electroweak condensate, and since the latter is also magnetically charged, it is repelled from the black hole by the magnetic force and therefore does not fall inside. However, it is not ejected further either, because it is attracted towards the black hole by gravitational force. It therefore remains trapped outside the black hole.
Our ring black holes, a new type, can be macroscopic in size, around a centimeter, while the elementary particles that make up their rings normally appear on the scale of the infinitesimally small.
Romain Gervalle and Mikhail Volkov, Provided by the author
As these black holes are described by experimentally confirmed theories, this strongly suggests that they exist not only as mathematical solutions, but also as real objects in the Universe.
Could we detect these black holes?
It is clear that these hairy black holes could not form today. On the other hand, the conditions favorable to their formation could have been encountered in the first moments of the Universe, in the extremely dense and fluctuating primordial plasma. These would therefore be primordial black holes.
Read more: Unexpected discovery of a link between black hole jets and their host galaxies
It is important to note that these black holes are stable, because the presence of the rings reduces the mass of the black hole, so getting rid of them would be energetically unfavorable. They could therefore manage to survive until today and become part of dark matter, this substance whose exact nature remains unknown to this day and which can only be detected by its gravitational influence. These hairy black holes could be detected by their interaction with rotating neutron stars (pulsars), because if they are absorbed by one of them (which can happen because they are much smaller and lighter) , then the star continues to exist with the black hole inside but this must abruptly change its rotation period, which could be detectable.
